The present invention relates to a magnetron for use in microwave ovens and the like, and more particularly to a mechanism for restraining the leakage of harmonic components from the output portion of the magnetron.
Generally, a magnetron for a microwave oven generates a microwave of 2.45 GHz as a fundamental wave. When generating the microwave, the magnetron generates harmonic components having frequencies of integral multiples of the fundamental wave in addition to the fundamental wave, simultaneously. When the harmonic components are radiated from the output portion, just like the fundamental wave, the harmonics are propagated into the microwave oven. Since the wavelengths of the harmonics are short, when they are propagated once into the microwave oven, it is difficult to prevent their leakage to the outside of the microwave oven. Since leakage power leaked to the outside of the microwave oven may cause wireless communication failures, the limit of the leakage is controlled by law in Japan.
Hence, in order to restrain harmonic components from generating from a magnetron itself inside a microwave oven, a magnetron provided with a quarter-wave choke at its output portion for outputting a microwave is generally used.
This kind of conventional magnetron will be described below referring to the accompanying drawings.
FIG. 10 is a sectional view showing the main portion of the conventional magnetron. FIG. 11 is a graph showing the noise levels of respective harmonics in a microwave oven in which the conventional magnetron is used. FIG. 12 is a graph showing an example wherein the noise levels in the frequency bands in the vicinity of a third harmonic leaked from the microwave oven in which the conventional magnetron is used is plotted in narrow ranges. In FIG. 11 and FIG. 12, the vertical axis represents noise level [dBpW], and the horizontal axis represents oscillation frequency [GHz].
As shown in FIG. 10, a plurality of anode segments 102 are secured to the inner wall of an anode cylinder 101, and these anode segments 102 are disposed so as to be directed toward the central axis of the anode cylinder 101. Inside the anode cylinder 101, a cathode 105 is disposed along the central axis thereof, and each of the upper and lower ends of the cathode 105 is secured to an end hat 106. The upper and lower ends of the respective anode segments 102 are connected alternately via a pair of large and small strap rings 103 and 104, respectively. At the upper and lower opening ends of the anode cylinder 101 having a cylindrical shape, metal cylinders 108 are heretically sealed via magnetic pole pieces 107.
In the upper portion inside the metal cylinder 108, that is, on the output side, a cylindrical choke 109 for restraining the third harmonic and a cylindrical choke 110 for restraining the fifth harmonic are disposed substantially coaxially. As shown in FIG. 10, one end of an antenna lead 113 is secured to one of the anode segments 102. This antenna lead 113 passes through the magnetic pole piece 107 and extends upward inside the metal cylinder 108 along the central axis thereof. The antenna lead 113 passes through the inside of the metal cylinder 108 and passes through an output portion 120 comprising a ceramic cylinder 111 and an exhaust pipe 112 so as not to make contact with the inner face thereof. The end of the antenna lead 113 is crimped and secured to the output portion 120 together with the exhaust pipe 112.
By using the conventional magnetron configured as described above for a microwave oven, the level of noise leaking from the microwave oven was measured. As shown in FIG. 11, among the noise levels of the respective harmonics of the fundamental wave (2.45 GHz), the level of the third harmonic, a 7.35 GHz band, was higher than the levels of the other harmonics.
FIG. 12 is a graph showing a result wherein the noise level of the third harmonic leaked from the microwave oven in which the conventional magnetron is used is plotted in narrow ranges. As shown in FIG. 12, in the vicinity of the third harmonic, the levels in a low side band of 6.9±0.15 GHz and a high side band of 8.3±0.15 GHz were high. More specifically, the noise level was about 80 dBpW at the third harmonic of 7.35 GHz, about 87 dBpW at the low side band of 6.9±0.15 GHz and about 95 dBpW at the high side band of 8.3±0.15 GHz.
As described above, the conventional magnetron had a choke structure to restrain the third and fifth harmonics. However, even when such a magnetron was used for a microwave oven, the restraint of the noise level of the third harmonic was still insufficient in comparison with the other harmonics as shown in the noise level graphs of FIGS. 11 and 12; in particular, in the low side band of 6.9±0.15 GHz and the high side band of 8.3±0.15 GHz in the frequency bands in the vicinity of the third harmonic, the magnetron had a problem of not producing the effect of the harmonic restraint chokes.